Conventionally, there has been known as an acoustic feedback prevention apparatus for use in a sound system, a apparatus for detecting acoustic feedback based on a sound signal input from a microphone and removing the acoustic feedback component by using a band rejection filter corresponding to an acoustic feedback frequency (see, for example, Patent Literature 1). However, in some cases, it was difficult to sufficiently remove the acoustic feedback component only by the use of the band rejection filter.
Accordingly, there has been proposed an acoustic feedback prevention apparatus which determines that acoustic feedback is independently occurring if a sound signal input from a microphone is in a soundless state (in the state without sound signal components except an acoustic feedback component) and mutes (deadens) the sound signal to be output (see, for example, Patent Literature 2). The acoustic feedback prevention apparatus determines the probability of acoustic feedback in every band based on the sound signal input from the microphone, lowers the signal level in those bands with a high probability of acoustic feedback with a notch filter, determines whether or not the input sound signal is under the soundless state based on the signal level filtered with the notch filter, and sets the level of the sound signal to be output at zero upon determination of the signal being under the soundless state.
However, in the conventional acoustic feedback suppression apparatus, when a sound signal in the same band as the band determined to have a high probability of acoustic feedback was input from the microphone while output sound signals are muted (deadened), the muted (deadened) state was not canceled and thereby a phenomenon of a speech head interruption (a phenomenon in which the first portion of a speech (head of speech) of a user is not amplified) might occur.